The present invention relates to a differential amplifier and, more particularly, to a class B push-pull differential amplifier of the single-end output type configured to have a single current amplifying stage.
A configuration as shown in FIG. 6 has been used for class B push-pull differential amplifiers of the single-end output type configured to have a single current amplifying stage.
As shown in FIG. 6, this configuration includes a first differential pair formed by n-p-n transistors Q1 and Q2 having a common emitter to which current bias is applied by a constant current source I1 and a second differential pair formed by n-p-n transistors Q3 and Q4 having a common emitter to which current bias is applied by a constant current source I2 and in which current mirror circuits respectively formed by p-n-p transistors Q5, Q6 and Q7, Q8 are respectively provided at collector outputs of each of the first and second differential pairs as active loads to form a differential current output. Further, emitter grounded current amplifying stages formed by p-n-p transistors Q9 and Q10 are provided as output current amplifying stages, and the differential current output from the preceding stage is connected to an input base end of each of the output current amplifying stages. In addition, the input of a current mirror circuit formed by n-p-n transistors Q11 and Q12 is connected to a collector thereof serving as an inverted current output, and the output of the same is connected to a collector serving as a non-inverted current output, thereby providing a single-end output. A capacity C1 is provided between the base and collector of the output current amplifying stage (the p-n-p transistor Q10) whose collector serves as an direct output to provide phase compensation for preventing oscillation.
FIG. 7 shows a configuration wherein a negative feedback is provided to the above-described differential amplifier to use it as a voltage amplifier. In FIG. 7, a negative power supply terminal VEE is set at a ground potential, and a voltage source VCC is provided at a positive power supply terminal VCC. Further, a voltage source VBIAS is provided at a non-inverted input IN(+) to apply current bias through a resistor RIN, and a feedback resistor RNF1 is provided between an output OUT and an inverted input IN(-). Furthermore, a signal voltage source VIN is connected to the non-inverted input IN(+) as an input through a DC interrupting capacity CIN; one end of a resistor RNF2 is connected to the inverted input IN(-) to set a voltage gain; and the other end of the resistor RNF2 is connected to the ground potential through a DC interrupting capacity CNF. Referring to the extraction of output, a load resistor RLOAD which is at the ground potential at one end thereof is connected to the output end through a DC interrupting capacity COUT to form a voltage amplifier.
The above-described class B push-pull differential amplifier has a first problem in that the phase compensation for preventing oscillation is not sufficiently effective. In the circuitry shown in FIG. 6, the capacity 1 for phase compensation is inserted only in one of the current amplifying stages. Therefore, when it is assumed that a negative feedback is to be provided, the phase margin can fall short to cause oscillation in a state wherein the current amplifying stage including the current mirror circuit primarily undertakes the current amplifying operation. This problem can be mitigated to some degree by adding a capacity (capacity C2 indicated by the broken line) between the base and collector of the other current amplifying stage to achieve phase compensation. However, since the collector is connected to the input of the current mirror circuit in which the voltage transition is compressed relative to the signal current, the amount of high frequency components reduced as a result of the feedback from the collector to the base will be insufficient. In order to perform compensation with a sufficient phase margin, a large capacity is required in this region. This can increase the capacity which is less suitable for integration into an IC, resulting in an increased chip size.
There is a second problem in that the above-described conventional class B push-pull differential amplifier is subjected to significant fluctuation of the bias current (idling current) at the output stage. In the circuitry shown in FIG. 6, the bias current at the output stage is determined as follows. When the bias currents of the transistors of the input differential pair are balanced in a non-signal state, an offset current equivalent to the base current of the active load current mirror circuit provided at the collector output thereof is produced at the output of this stage. As a result, the current at the input base end of the emitter grounded current amplifying stage provided at the next stage is biased by the offset current from the preceding stage. Therefore, if the transistors forming the current mirror circuit at the input are matched with the transistors forming the output current amplifying stage, the current at the output current amplifying stage is biased by a current equivalent to the bias current of the input current mirror circuit.
Although a current gain inherent to the transistors is maintained at the current amplifying stage, variation of the bias current at the output stage during manufacture tends to increase when it is integrated into an IC because the bias current of the output stage depends on the matching between the current gains of the transistors. When the bias is extremely small, crossover distortion unique to a class B differential amplifier can occur.